Lead as a contaminant is often found in the soils around lead smelters, battery breaking/recycling facilities, incinerator ash facilities and foundries including metal and leaded gasoline manufacturing plants. Contamination occurs when lead-containing chemicals are used in the plants, and waste containing the lead is allowed to spill over or drain into the soil. Many abandoned hazardous waste sites are heavily contaminated with lead, threatening human health, the food chain, the ecosystem and the environment. Federal legislation, such as the National Contingency Plan (NCP), the Superfund Act (CERCLA) and the Superfund Amendments Reauthorization Act (SARA) specify the remediation of sites containing lead-toxic soils and solid wastes.
The Resource Conservation and Recovery Act of 1976, commonly known as the RCRA, provided for federal classification of hazardous waste. The statutory language defines "hazardous waste" as solid waste or combinations of solid waste which pose a "substantial present or potential hazard...when improperly treated, stored, transported, or disposed of, or otherwise mismanaged." Any solid waste that exhibits one of the hazard characteristics defined in subpart C of Part 261, Volume 40, Code of Federal Regulations is, by definition, a hazardous waste.
A solid waste is considered to be a hazardous waste if it is listed, or it exhibits characteristics of either ignitability, corrosivity, reactivity, or toxicity as determined by the Toxicity Characteristic Leaching Procedure (TCLP) (USEPA Method 1311). Historically, toxicity characteristic regulations had been based on the Extraction Procedure (EP) Toxicity Test (USEPA Method 1310), which specified laboratory steps to be followed in analyzing samples. The test was aimed at identifying the tendency of wastes to generate a leachate with concentrations of contaminants greater than the values listed in Appendix II of the Code of Federal Regulations, Part 261.24, page 406, revised July 1, 1988. If concentrations of leachable, mobile lead were found to be greater than 5 milligrams per liter, the material was considered characteristically toxic for lead and hence hazardous with respect to lead content. Such characteristically toxic wastes required treatment to comply with the USEPA regulations defining the treatment standards for lead and other parameters of concern. This EP Toxicity Test is now obsolete, and has been replaced by the TCLP for 39 different parameters including lead.
Effective Nov. 8, 1990, USEPA established the treatment standard for lead wastes (D008), and particularly for lead contaminated soils and solid wastes, at a toxicity characteristic level of 5 milligrams per liter in the extraction fluid according to the TCLP Test. The TCLP Test is much more rigorous--and is more uniformly applicable to a larger number of parameters--than the EP Toxicity Test. It replaced the EP toxicity method for RCRA waste determination. The TCLP Test requires sizing of waste material to less than 3/8 inches or 9.5 mm and agitation of a 100g waste sample in 2 liters of specified extraction fluid for 18 hours on a rotating agitator at a speed of about 30 revolutions per minute. The lead concentration is determined in the extraction fluid after filtration under pressure, and expressed in units of milligrams per liter (mg/l).
Any solid waste that contains leachable TCLP lead levels in excess of 5 milligrams per liter is considered characteristically toxic and hence hazardous. Such hazardous waste must be treated with at least one of the Best Demonstrated Available Technologies (BDAT) and/or with an alternative technology to decharacterize the waste for lead toxicity. In other words, treatment of the lead-bearing waste with a BDAT for decreasing TCLP lead to a level below 5 mg/l is required before land disposal is permitted. Land disposal methods include waste staging on a land surface, placing waste into a landfill, using surface impoundment techniques, waste piling, disposing of waste in injection wells or land treatment facilities (land farming), or impounding the waste in salt domes, salt bed formations, underground mines or caves, and bunkering the waste in concrete vaults. Land disposal restrictions ban treated wastes with TCLP lead levels greater than 5 mg/l in the leachate. Such characteristic lead toxic wastes must be treated with a cost effective and practical technology that is commercially available and that provides substantial treatment, and that beneficially results in a decrease in risk to human health and the environment.
Prior to the present invention for treatment of contaminated soils and D008 solid wastes, there existed no technology that could be applied (i) cost effectively on a commercial scale to treat lead-toxic soils and solid wastes, (ii) to decrease the waste volume and at the same time work under substantially dry conditions with no generation of wastewater or other byproducts, (iii) to comply with the latest and final land ban regulations (55 Fed. Reg. 22693-94 (1990)), (iv) to cure the wastes in few hours for rapid sampling and final internment evaluation and (v) to a wide variety of lead-toxic solid wastes, soils and sludges with a tremendous flexibility of scale and mobility.
Various conventional methods have been tried to remove leachable, mobile lead from soils and solid waste materials. Those methods include washing, leaching and extracting the lead. According to conventional practice, contaminated soil or solid waste material is excavated from the ground for processing and/or washing. During washing, the contaminated material is immersed or supersaturated in water or other specified solutions while it is being agitated. Removal of lead from contaminated soils and solid wastes by leaching, extraction and/or washing procedures is extremely expensive and cost-prohibitive because this method generates vast quantities of lead-toxic wastewater which requires further treatment and disposal.
As understood by the inventors, none of the earlier processes reduced the TCLP lead content to below 5 mg/l of lead in the extract from lead contaminated soil or solid waste material.
Wet methods for removing lead from contaminated soil or solid waste involve the use of water in the formation of slurries, which require cumbersome equipment for the separation of lead from the waste material. The separated solids are usually wet, so that the end product fails the Paint Filter Test (USEPA Method 9095 under SW-846). Further processing of these wet materials is therefore required before disposal as a stabilized material. The additional steps required in the treatment by conventional wet processing of contaminated soil and solid waste are prohibitively expensive.
Other conventional techniques involve the chemical fixation of lead in contaminated soils and solid waste. One well-known technique according to the International Technical Information Institute involves the extraction of lead using nitric acid and/or aqua regia, and a subsequent purging of the resultant lead nitrate solution with hydrogen sulfide gas to precipitate the lead nitrate as lead sulfide. The use of noxious hydrogen sulfide gas, however, necessitates specific health and safety measures that increase environmental remediation costs.
Falk et al. U.S. Pat. No. 4,687,373, describes a composition which encapsulates contaminants such as lead in soils, sludges, sediment and ash. A cementitious matrix in the form of metal metasilicates is formed to encapsulate the contaminants. The metal metasilicates, however, detrimentally increase the volume and weight of the treated soil or solid waste material.
Hemwall U S. Pat. No. 3,201,268, describes a method for stabilizing clay soils by mixing phosphoric acid, or a combination of phosphoric and sulfuric acids, with the soil. This mixture is further combined with a water-soluble lead salt. The resulting composition may be compacted and cured to produce a stabilized mass strength as compared with untreated soil. This method is suitable for the stabilization of argillaceous soils and clays containing aggregates.
Webster et al. U.S. Pat. No. 4,028,130, describes a method for disposing of municipal sewage plant waste materials, particularly digestive sewage sludge. The sludge is treated with cementitious reactants, including calcium sulfate, to form a hardened product for subsequent disposal. The sewage sludge contains heavy metals, such as lead, which may be involved in the cementitious reaction.
Gouvenot U.S. Pat. No. 4,615,643, describes a method of sealing a mass of stored waste containing heavy metal cations in soil. A grout is added to the soil which comprises cement, clay, silicate, sodium carbonate and an alkali-metal pyrophosphate or tartrate. The lead cation forms a water-insoluble compound upon reaction with the sodium carbonate and pyrophosphate. The process requires a long curing time and has limited commercial application.
Stanforth U.S. Pat. No. 4,889,640, discloses a method of fixation using reactive calcium carbonate, reactive magnesium carbonate, and reactive calcium magnesium carbonate for reaction with lead and cadmium in hazardous solid wastes (pH 6 to 9) from foundries and metal operations. Stanforth's technique reduces EP Toxicity Test lead and cadmium in hazardous wastes when treated with a water-softening lime sludge (a source for reactive carbonates of calcium and magnesium). Stanforth cited the work of Inglis (U.S. Pat. No. 4,652,381) in which crystalline and non-reactive forms of calcium carbonate (such as limestone) were mixed with a highly acidic wastewater (pH 2) to precipitate lead, copper and zinc out with formation of a sludge that might contain significant amounts of leachable metals. This solid waste sludge requires further treatment in order to render it non-hazardous prior to its disposal. As recognized in Stanforth, however, limestone (calcium carbonate) is relatively inefficient at removing heavy metals such as lead and cadmium from hazardous solid or sludge waste because of the slow release of carbonates to react with heavy metals. Furthermore, it is common knowledge that carbonates decompose under acid conditions and liberate carbon dioxide as well as metals that may endanger the environment in the presence of acid rain or landfill leachate.
Bonee U.S. Pat. No. 4,701,219, discloses the treatment of spent sorbent wastes (containing leachable vanadium, nickel, and sodium) with alkaline earth metal compounds, including calcium sulfate. According to that patent, powdered lime (calcium hydroxide or calcium oxide) and calcium fluoride were most effective in decreasing the leachable vanadium and nickel.
Douglas et al. U.S. Pat. No. 4,671,882, discloses the generation of non-hazardous sludge from wastewater containing a mixture of metals by first adding phosphoric acid to lower the pH of the wastewater less than 5.0. Thereafter, the acidified wastewater was treated with a coagulant, ferric chloride, and the pH was raised to a range of 7 to 8.5 with calcium hydroxide. An anionic polymer (DREW FLOC 270) was employed as a flocculent to aid dewatering of the sludge. The resulting sludge contained heavy metals in non-leachable form by EP Toxicity Test criteria. The method developed by Douglas et al. for industrial wastewater treatment creates sludge that is characteristically non-hazardous by EP toxicity criteria for zinc, lead, chromium, nickel, copper and cadmium, but which may not pass the Paint Filter Test and the TCLP criteria. There is no disclosure of TCLP testing conducted on this sludge or wastewater. The method disclosed by Douglas et al. does, however, generate a supernatant wastewater stream containing 1 ppm lead as compared to a standard limit of 0.05 ppm lead for drinking water under the Safe Drinking Water Act (SDWA), therefore requiring further treatment of the supernatant during dewatering operations for removal of residual metals. The method disclosed in Douglas et al. does not appear to be transferable to lead contaminated soils or solid wastes at any scale.
The conventional processes as described above typically do not reduce levels of leachable lead below the maximum concentration of contaminant allowed under current land ban regulations as per the TCLP Test. Moreover, some of the conventional methods involve wet processing, which is burdensome, cost prohibitive, and requires a considerable amount of equipment to separate the lead from the contaminated soil or solid waste material in addition to treatment steps.
An innovative and cost-efficient technology is therefore needed that does not generate wastewater or supernatant and that quickly treats the lead-toxic soils and D008 solid wastes under relatively dry conditions while fixing the leachable lead to levels below 5 mg/l by TCLP Test criteria as required under EPA regulations.
Solidification methods based on cementation technology require at least 28 days of curing time, increase the waste volume, and may raise the pH to a range from 12.5 to 13.5. Hardened concrete material is not conducive to retreatment in the event treatment fails TCLP confirmatory testing. Solidification methods utilizing lime kiln dust, calcium carbonate and/or powdered lime for lead fixation are temporary solutions for lead treatment. Furthermore, those methods increase the waste volume and mass, and therefore, dilute the lead in the final waste matrix.
The use of phosphoric acid alone for fixing lead in solid waste and soils fails to pass the TCLP lead criteria in many cases for lead contaminated soils. Addition of gypsum powder to phosphoric acid treated soils, however, further lowers the TCLP lead levels below the regulatory threshold as illustrated in the examples herein. Stabilization of lead in D008 soils and solid waste is crucial, and a BDAT is therefore urgently needed that is (a) relatively simple and feasible for treating hazardous solid wastes; (b) commercially practicable, (c) economically applicable and transferable to different lead contaminated sites, (d) rapid, and (e) free of side streams or byproduct wastes. The technology disclosed herein generates an end product that is easily handled and that passes the Paint Filter Test used for solid waste.